A variety of small followups to splitting the request objects into separate files:
* Make PurpleRequestField into an abstract type and drop the field type enum.
* Add request files to GObject introspection.
* Replace purple_debug with GLib logging.
* Remove extra #include where possible.
* Remove most early typedef which were there for #include loops.
* Put G_DEFINE_TYPE in a consistent place.
* Add Since tags to objects since they are new (but not the functions), and also to all of PurpleRequestPage/PurpleRequestGroup which were fully renamed.
* Use website for GNU reference in license header.
* Clean up old entries in ChangeLog.API.

Compiled, then checked Class Hierarchy in docs and confirmed that the structs were now there.